The invention relates to a process and an installation for the treatment, by drying and/or heating by fluidisation and/or gas conveying, of a combustible granular and/or pulverulent material, such as a coal which is to be dried and/or pre-heated for charging low-coking into coke ovens by the so-called dry charging and preheated charging methods, or any other application in which a subsequent physical or chemical treatment is intended.
Processes and installations of the type which the invention proposes to improve are described, for example, in U.S. Pat. Nos. 3,884,620; 4,008,042; 4,174,947 and U.K. patent specification No. 1,546,294.
The preheating of coal fines requires the use of flue gas which is free from oxygen, at a temperature which must not exceed approximately 500.degree. to 800.degree. C. To achieve this result, it has been proposed to recycle a part of the flue gas which has been used for the preheating into the combustion chamber, where a neutral combustion is carried out.
To derive the greatest advantage from dry charging or preheated charging, the coal particles must be finely ground, with, for example, 80% and even 90% smaller than 2 mm. These fine particles of coal, preheated to 250.degree. C., are separated from the conveying fluid by means of two cyclone stages, whose collecting efficiency is 98 to 99%.
The system of treatment of the flue gas, as presented, makes it possible to achieve the desired result but does not produce advantages only. In fact, the recycled flue gas wich has not been purified and is thus still charged with fine dust is introduced, by means of a fan, into the combustion chamber, in the vicinity of the burner which operates by stoichiometric combustion and hence at a temperature close to the theoretical temperature which, for coke oven gas, exceeds 2,000.degree. C. The fine particles of coal may burn or melt, which is undesirable. In the known devices, the flue gas at the cyclone exit must be purified before release into the atmosphere.
In a first type, as described in U.S. Pat. No. 4,174,947, a dry dust removal is carried out only on the discharged flue gas, whilst in a second type, as described in U.S. Pat. No. 3,884,620, the whole of the flue gas, whether recycled or not, is subjected to a dry dust removal.
Furthermore, when the installation is idle (for example in the event of a breakdown upstream or downstream of the preheater), the amount of flue gas discharged into the atmosphere becomes very low so that, in the case of the first type of final dust remover, located on the line leading to the atmosphere, the working rate drops to a zero or very low throughput. If the dust remover is of the wet type, for example a high energy venturi, the reduction in the flue gas throughput is not troublesome. However, a wet dust remover is inappropriate because the recovered dust is advantageously introduced into the coke fines. With a dust remover of the dry type (sleeve-type filter and electrostatic filter) the reduction in the throughput of flue gas causes condensation and hence corrosion in the apparatus. Furthermore, there is the danger that the dust remover may run at sub-atmospheric pressure, thus causing stray entry of air into the installation.
However, if it is desired to instal a dry dust remover into an installation of the second type, it is necessary that the dust remover shoulder be of a size which can treat the maximum throughput of flue gas corresponding to normal running. Since it is not necessary to remove dust completely from the flue gas recycled to the combustion chamber although this is desirable, it is necessary to provide a very expensive installation suitable for the maximum throughput of flue gas. Furthermore, if the flue gas is purified dry, at least before being discharged into the atmosphere, the recovered dust should advantageously be reintroduced into the coal fines, that is to say into the coal which is undergoing, or has undergone, the prior treatment. However, this recovered dust consists of very reactive dry particles which may ignite spontaneously in contact with air, and which fly very easily. It is thus necessary to treat the dust so as to render it inert and capable of being handled. It is very difficult to wet with water. Furthermore, moistening it reduces the value of the dry dust removal process. It has already been proposed, in French Patent No. 1,265,397, or equivalent U.K. Patent 963,435 to improve the quality of the steelmaking coke obtained from a dried coal by adding to the coal, before or after it is dried, a hydrocarbon binder consisting of a coal tar, an artificial tar or a bitumen, or an extract or extraction residue, the said hydrocarbon binder being added in such a way that it assumes a pasty or liquid consistency. Another known advantage of the introduction of a hydrocarbon binder is to reduce the amount of flying dust at the time of charging the coal into the coke oven. It is for the same reason that it has also been proposed, in French Patent No. 2,306,252, or equivalent U.S. Pat. No. 4,030,983, to add the hydrocarbon binder in one of the conveyors for the already preheated coal and to do so just before the coal is charged into the coke oven, so as to reduce the flying dust during charging.
The main object of the invention is to avoid the disadvantages of the two abovementioned types of installations comprising a dry dust removal and to provide a process and an installation which possesses a dry dust remover which is neither overloaded nor oversized, and which is not subject to corrosion due to the danger of an insufficient throughput causing the temperature to drop below the dew point, especially during the idle periods when little heat is being produced in the installation. In other words, it is an aim of the invention to achieve the optimum dust removal efficiency of the dust remover.
Another object of the invention is to provide a solution to the problem of recovery and of handling and re-use of the recovered dust, by means of a particular treatment which will be presented.